For in situ Raman monitoring and other applications, there is a need to couple elongated end optics to probe heads used to deliver laser excitation and collect sample spectra. However, quality issues arise with longer probe lengths. In particular, it is difficult to maintain alignment of the beam path from the probe head through long, thin immersion end optics. Tolerances on straightness and stiffness of the optic tube are difficult to maintain, resulting of vignetting of the beam at the focusing objective at the distal end of the probe, especially under mechanical stress. Such problems are further exacerbated by the beam divergence inherent in Raman probe heads that are coupled to an analyzer via large-core multimode optical fibers.
FIG. 1 is a cross sectional diagram of an MR-type filtered fiber-coupled Raman probe head 100 available from Kaiser Optical Systems, Inc. of Ann Arbor, Mich. The probe head 100 interfaces via collet 110 to end optic tube 101. Different end optics are available in various lengths and focal depths for different applications including in situ insertion/immersion Raman analysis of reaction vessels, automated laboratory reactors, extruders, process streams, and so forth.
As described in U.S. Pat. No. 6,907,149, incorporated herein by reference, laser excitation is brought into the probe head 100 via fiber 102, which is then collimated by lens 104. The collimated light then passes through a bandpass filter 108 to remove non-laser wavelengths generated en route from the source. The filtered light is reflected by a mirror 106 onto a beam combiner 120 which then enters the end optic tube 101 as a counter-propagating beam 122. The light scattered by the sample beyond the distal end of end optic tube 101 returns along the same counter-propagating beam path 122, passes through beam combiner 120 in the reverse direction, and is filtered by an optional notch filter (not shown) before being focused by lens 114 onto the end of collection fiber 112.
FIG. 2 is a simplified diagram of the probe head end optic of FIG. 1. Due to the beam distribution and filtering functions in the probe head, the probe head is typically larger than an allowable sample interface, which may require tubes of varying lengths and/or sizes for immersion in various sample vessels. Note that while “tubes” are used with reference to sealed immersion optics, the concept can be generalized to any suitable support structure for the optics. Input optical fiber 200 has a high numerical aperture (NA) at the edges of the fiber core, which is focused at infinity by collimating lens 206, establishing the excitation-collection beam 210 within the tube 212 of the end optic. The distal end of the optic includes a focus lens 202 and window lens objective assembly 204, which together form an image of the fiber at a focal point 220 just outside the window 204 within the sample (i.e., sample focus 220).
The divergence of the beam within the probe tube 212 is determined by the diameter of the fiber core 200 and the focal length of collimating lens 206. With shorter tubes, the beam size expansion due to divergence is limited, and the entire beam path is transmitted efficiently into the sample at focus 220. However, some applications require longer tubes, including tubes with lengths of 500 mm or more. At such lengths, as depicted in FIG. 3, divergence of the beam at 302 causes the optical path from the probe head to “overfill” focus lens 202, which in turn results in the beam path being blocked with less efficient focusing or vignetting at sample focus 320 within the sample.
The need remains, therefore, for a solution to the problem of vignetting in elongated end optics for in situ spectroscopic probes.